Absorbed electrolyte batteries are available in a variety of configurations. A well known example of such batteries is the sealed lead-acid type, although other types of absorbed electrolyte batteries are available, such as the nickel-cadmium configuration. Sealed lead-acid batteries, as an example of the type under consideration, typically have certain features in common. A common gas manifold system interconnects all cells and a venting device is normally provided to prevent excess gas pressure buildup within the battery. The battery elements are housed within a rectangular container which is divided by partition walls into a series of cells. An electrode stack is closely fitted within each cell. The electrode stacks comprise alternate positive and negative plates with separators interposed between the positive and negative plates.
In sealed lead-acid batteries, there is substantially no free unabsorbed electrolyte in the cells. The major portion of the electrolyte is restrained in the highly absorbent microfine glass fiber separator material between the positive and negative plates and within the pores of the positive and negative active material of the plates.
Although electrolyte is immobilized and absorbed in special separators, the separators are not fully saturated so that the gases evolved during charging or at other times can diffuse rapidly from one electrode to the other. Thus, in what is termed an "oxygen cycle," oxygen is produced at the positive electrode and diffuses to the negative electrode where it rapidly reacts to combine with active lead. Effectively, this reaction partially discharges the negative electrode, preventing the negative electrode from reaching its fully charged state, thereby minimizing the evolution of hydrogen. When the oxygen reacting at the negative electrode is equal to or greater than the rate of oxygen being produced at the positive electrode, water loss through electrolysis and, more importantly, pressure build up are minimized.
However, the oxygen cycle takes place only under the following conditions. First, both the positive and the negative plates must be in intimate contact with the separator material so that the entire surface of the plates has adequate electrolyte for its electrochemical requirements. Thus, it is of paramount importance that the cells be maintained under a compressive force to insure the necessary intimate contact between the plates and separators. Also, the oxygen initially produced at the positive plates must be contained in the cells under pressure (typically 0.5 to 8.0 psig) so that it contacts the negative plates to effect the oxygen cycle.
In conventional sealed lead-acid batteries, the distance between the partition walls corresponds to the thickness of the electrode stack such that the stack closely fits within the cell to achieve the desired compressive contact between the negative and positive electrode plates and the separators.
Unfortunately, the elevated internal pressures necessary to ensure the oxygen cycle occurs in combination with the additional increase in pressure from the resulting oxygen cycle, cause conventional containers to bulge, thus causing a relaxation of the compressive force in the end cells. Consequently, the intimate surface contact between the separators and positive and negative plates in the end cells is reduced causing battery efficiency to be significantly reduced.
Techniques used to prevent end wall bulging have taken many forms. For example, U.S. Pat. No. 5,187,031 discloses an anti-vibration plaque for insertion between each end cell and its corresponding end wall. The anti-vibration plaques are inserted into the battery container during manufacture and are designed to protect the end cell elements from the effects of vibration. Although this art may prevent end wall bulging in limited circumstances, it fails to fully reinforce the battery container end walls. Because the vibration plaque and end wall do not function unitarily, internal pressure may separate the plaque and end wall thus allowing the end wall to deform.
U.S. Pat. No. 4,729,933 discloses various batteries designed to maintain the element containing cells under compressive force by providing an auxiliary compressive means, at least a portion of which is spaced away from the end wall to form an auxiliary cell. Importantly, the pressure within the battery container as bounded by the end walls, i.e., the active cells and the auxiliary cells, is uniform. The '933 patent asserts that while the end walls may bulge, the auxiliary compression means maintains pressure on the active cells. By design, the '933 battery sacrifices the cosmetic appearance to maintain the function of the battery.
Other sealed lead-acid batteries presently being produced attempt to nullify bulging through the use of stiffer material less prone to bulge, stiffening ribs incorporated in the container end wall design, or thicker container end wall construction. Existing containers may also be molded with extremely thick end walls using a filled material to increase stiffness. Filled materials are more expensive than battery grade plastics. Furthermore, filled materials are typically more brittle than battery grade plastics and cause the battery container to be more susceptible to damage. Additionally, filled materials inhibit recycling because the filled material does not float in the recycling process and is, therefore, not readily separated from the polymeric material of the container.
Another approach involves inserts that are bonded and/or locked to the outside wall to prevent gas pressure between the wall and the stiffener to bulge the end wall. For example, batteries have included an insert molded with dovetails that match dovetails molded into the container end walls, as are disclosed in copending application Ser. No. 08/961,617, which is likewise assigned to the assignee of this application. Inserted stiffeners with dovetails must match the mating dovetails on the container requiring extremely close tolerances. Batteries incorporating such inserts on the inside surface, however, can still exhibit distortion due to the pressure build-up between the wall and the insert. Further, slip-in inserts located on the outside of the container trap acid and wash water. Batteries incorporating such inserts on the inside surface, however, can still exhibit distortion due to pressure build up between the wall and the insert.
While these approaches may offer some improvement, none of these techniques are entirely effective. For instance, thick and thin walls create very difficult molding parameters. Not only do such configurations increase battery manufacturing costs because material costs and molding cycle time are higher, but they also increase the weight of the batteries, another important consideration in battery design.